Blood coagulation is the primary defense mechanism of the host against excessive blood loss and is triggered in a wide variety of diseases at local or disseminated sites. Fibrinolysis is the compensatory mechanism for dissolution of the fibrin clots formed as a result of blood coagulation. Therefore, fibrinolysis is essential for restoring integrity to extravascular and intravascular sites of fibrin deposition, for maintaining vascular patency, and for re-establishing normal hemostasis. In vivo, fibrinolysis is achieved by the classical fibrinolytic pathway with plasmin functioning as the primary enzyme and by the alternative fibrinolytic pathway in which leukocyte elastase and cathepsin G are the major fibrinolytic proteases. The activities of both fibrinolytic pathways are regulated by specific inhibitors, with alpha 2-antiplasmin being the primary inhibitor of plasmin and alpha 1-antitrypsin and alpha 1-anti-chymotrypsin being the primary inhibitors of elastase and cathepsin G, respectively. Efficient and well-regulated fibrinolysis is achieved by the balance of the activity and the inhibitory capacities of each pathway and by the participation of both pathways in the fibrinolytic process. Pathological fibrinolysis occurs as a result of imbalances within or between the fibrinolytic pathways. This proposal focuses primarily upon the alternative fibrinolytic pathway. We will seek to elucidate mechanisms by which this pathway is activated, develop specific immunoassays for the components of this system including the enzymes, the inhibitors, and the enzyme: inhibitor complexes, and to discriminate the fibrinolytic products generated by this system. The ultimate goal will be to establish the contribution of the alternative fibrinolytic pathway in normal hemostasis and in pathological conditions. Selected aspects of the classical fibrinolytic pathway will also be investigated. We shall examine the cellular distribution and the functional heterogeneity of alpha 2-antiplasmin, and the structural basis for the interaction of plasmin with its primary substrate and its primary inhibitor. These studies should lead to a fundamental understanding of the specific mechanisms and events associated with each fibrinolytic pathway and permit assessment of the relative contributions of both pathways in normal and pathological situations.